Improved sulfur-cured rubber-base solid propellant containing magnesium oxide



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3,010,814 IMPROVED SULFUR-CURE!) RUBBER-BASE SOLID PROPELLANT CONTAINING MAG- NEIUM OXIDE Donald E. Carr, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Mar. 19, 1956, Ser. No. 572,195

14 Claims. (Cl. 52-.5)

This invention relates to improved solid propellent compositions and to a method for their preparation. In one of its more specific aspects this invention rel-ates to solid propellants having improved aging characteristics. In another of its more specific aspects this invention relates to a method for preserving high values of tensile strength, elongation and other mechanical properties in a solid propellent composition.

Recently it has been discovered that superior solid propellent materials are obtained comprising a solid oxidant such as ammonium nitrate or ammonium perchlorate, and a rubbery material such as a copolymer of butadiene and a vinylpyridine or other substituted heterocyclic nitrogen base compound, which after incorporation is cured by a quaternization reaction or a vulcanization reaction. Solid propellent mixtures and a process for their produc tion are disclosed and claimed in copending application Serial No. 284,447, filed April 25, 1952, by W. B. Reynolds and I. E. Pritchard.

In the production of such solid propellent compositions, it is desirable that the products have good mechanical properties such as high values of elongation and tensile strength, low modulus characteristics, as well as good burning rates and that these mechanical properties obtain in the cured propellent composition even after a substantial aging period. it is inevitable that these solid propellent compositions will be maintained in storage for varying lengths of time and it is of particular importance that their physical and ballistic characteristics remain unaltered in so far as it is possible.

It is an object of this invention to provide solid propellent compositions which have tensile strength and elongation values which remain high after aging. It is also an object of this invention to provide solid propellent compositions Whose burning rate remains substantially constant during an aging or storing period and which show a pronounced decrease in smoke evolution. It is also an object of this invention to provide a method for pre paring a solid propellent composition whose mechanical characteristics and burning rate are substantially unaffected by extended periods of storage. -It is also an object of this invention to provide a method for preparing a solid propellant composition displaying a decrease in smoke evolution. Other objects and advantages will be apparent to one skilled in the art upon study of this invention.

Broadly the invention comprises a solid propellent composition comprising a solid oxidizer, a binder containing a rubbery copolymer of a conjugated diene with a polymerizable heterocyclic nitrogen base of the pyridine series, compounding agents for curing the binder composition, a burning rate catalyst, and a minor amount of magnesium oxide. The invention also contemplates a method for the preparation of the solid propellent composition. I have discovered that propellent compositions, as above described, possess superior aging characteristics in that tensile strength and elongation values remain high, the burning rate is only slightly altered and in addition such compositions display a pronounced decrease in smoke evolution upon being fired. The amount of magnesium oxide added will usually be in the range of 0.25 to 5 parts by weight per 100 parts by Weight of oxidizer-binder mixture. The amount of magnesium oxide can be decreased or increased in order to obtain the desired result. Particularly beneficial results are obtained when zinc oxide is present in the propellent grain. Zinc oxide is ordinarily employed in the binder recipe as a curing agent.

The rubbery polymers employed as bindersin the solid propellent compositions of this invention are copolyrners of conjugated dienes with polymerizable heterocyclic nitrogen bases of the pyridine series. These copolyrners can vary in consistency from very soft rubbers, i.e., materials which are soft at room temperature but will show retraction when relaxed, to those having a Mooney value (Mk4) up to 100. The rubbery copolyrners most frequently preferred have Mooney values in the range between 10 and 40. They may be prepared by any polymerization methods known in the art, e.g., mass or emulsion polymerization. One convenient method for preparing these copolyrners is by emulsion polymerization at temperatures in the range between 0 and F. Recipes such as the iron pyrophosphatehydroperoxide, either sugar-free or containing sugar, the sulfoxylate, and the persul'fate recipes are among those which are applicable. It is advantageousto polymerize to high conversion as the unreacted vinylpyridine monomer is diificult to remove by stripping.

The conjugated clienes employed are those containing from 4 to 6 carbon atoms per molecule and include 1,3- but-adiene, isoprene, 2-metl1yI-L3-bu-tadiene, 2,3-dimethyl- 1,3-but-adiene, and the like. Various alkoxy, such as methoxy and ethoxy and cyano derivatives of these conjugated dienes, are also applicable. Thus, other dienes, such as phenylbutadiene, 2,3-dimethyl-l,3-hexadiene, 2- rnethoxy-3-ethylbutadiene, 2-ethoxy-3-ethyl-1,3-hexadiene, 2-cyano-1,3-butadiene, are also applicable in the preparation of the polymeric binders of this invention.

Instead of usinga single conjugated diene, a mixture of conjugated dienes can be employed. Thus, a mixture of 1,3-butadiene and isoprene can be employed as the conjugated diene portion of the monomer system.

The polymerizable heterocyclic nitrogen bases which are applicable for the production of the polymeric materials 'are those of the pyridine, quinoline, and isoquino line series which are copolymerizable with a conjugated diene and contain one, and only one,

substituent wherein R is either hydrogen or a methyl group. That is, the substituent is either a vinyl or an Various substituted deriva- Wlllfile R is selected from the group-consisting of hydrogen, alkyl, vinyl, alpha-methylvinyl, alkoxy, halo, hydroxy, cyano, iaryloxy, aryl, and combinations of these groups such as haloalkyl, alkylaryl, hydroxyaryl, and the like; one and only one of the said groups being selected from the group consisting of vinyl and alpha-methylvinyl; and the total number of carbon atoms in the nuclear substituted groups being not greater than 15. Examples of such compounds are 2-vinylpyridine;

2-vinyl-5 -ethylpyridine;

Z-methyl-S -vinylpyridineg 4-vinylpyridine 2, 3 ,4-trimethyl-5 -vinylpyri dine;

3 ,4-,5 ,6-tetramethyl-2-vinylpyri dine;

3 -ethyl-5-vinylpyridine;

2, 6-diethyl-4-vinylpyri dine; 2-isopropyl-4-nonyl-5-vinylpyridine; 2-methyl-5-undecyl-3-vinylpyridine; 2,4-dimethyl-5,6-dipentyl-3 -vinylpyridine; Z-decyl-S alpha-methylvinyl pyridine; 2-vinyl-3 -methyl-5-ethylpyridine; 2-methoxy-4-chloro-6-vinylpyridine;

3 -vinyl 5 -ethoxypyri dine; 2-vinyl-4,S-dichloropyridine;

2( alpha-methylvinyl) -4-hydroxy-6-cyanopyridine; 2-vinyl-4-phenoxy-5-methylpyridine; Z-cyano-S- (alpha-methylvinyl pyridine; 3-vinyl-5-phenylpyridine;

2- para-methylphenyl) -3 -vinyl-4-methylpyridine 3-viny1-5-( hydroxyphenyl -pyridine; 2-vinylquinoline; 2-vinyl-4-ethylquinoline; 3-vinyl-6,7-di-n-propylquinoline; 2-methyl-4-nonyl-6-vinylquinoline;

4 alpha-methylvinyl -8-dodecylquinoline; 3-viny1isoquinoline; l,6-dimethyl-3-vinylisoquinoline; 2-vinyl-4-benzylquinoline;

3 -vinyl-5 -ohloroethylquinoline; 3'-vinyl-5,6dichloroisoquinoline; 2-vinyl-6-ethoxy-7-methylquinoline;

3 -vinyl-6-hydroxymethylisoquinoline; and the like.

Oxidants which are applicable in the solid propellent compositions of this invention include ammonium, alkali metal, and alkaline earth metal salts of nitric, perchloric, and chloric acids, and mixtures thereof. Ammonium nitrate and ammonium perchlorate are the preferred oxidants for use in the solid rocket fuels of this invention. Specific oxidants include sodium nitrate, potassium perchlorate, lithium chlorate, calcium nitrate, barium perchlorate, and strontium chlorate. Mixtures of oxidants are also applicable. In the preparation of the solid rocket propellent compositions, the oxidants are powdered to sizes preferably 300 to 10 microns average particle size. The amount of solid oxidant employed is usually a major amount of the total composition and is generally in the range between 50 and 90 percent by weight of the total mixture of oxidant and binder. If desired,

however, less than 50 percent by weight of the oxidant can be used.

Combustion rate catalysts applicable in the invention include ammonium dichromate, iron oxide, potassium dichromate and metal ferrocyanides and ferricyanides. Ferric ferrocyanides, such as Prussian, Berlin, Hamburg, Chinese, Paris, and milori blue, soluble ferric ferrocyanide, such as soluble Berlin or Prussian blue which contains potassium ferric ferrocyanide, and ferric ferrocyanide which has been treated with ammonia, are among the materials which can be used. Ferrous ferricyanide, Turnbulls blue, is also applicable. A particularly effective burning rate catalyst is milori blue which is pigment similar to Prussian blue but having a red tint and is prepared by the oxidation of a paste of potassium ferrocyanide and ferrous sulfate. Other metal compounds such as nickel and copper ferrocyanides can also be employed. The amount of burning rate catalyst used, in the propellent compositions of this invention, are usually in the range of 0 to 15 parts by weight per 100 parts of oxidant and binder mixture with 1 to 4 parts per 100 parts of oxidant and binder mixture being preferred.

The binder contains a rubbery copolymer of the type hereinbefore described and, in addition, there may be present one or more reinforcing agents, plasticizers, wetting agents, and antioxidants. Other ingredients which are employed for sulfur vulcanization include a vulcanization accelerator, a vulcanizing agent such as sulfur, and an accelerator activator, such as zinc oxide.

A general formulation for binder composition prepared by sulfur vulcanization is given below:

Parts by weight Rubbery copolymer (as hereinbefore described) 100 Reinforcing agent 0-50 Pl'asticizer 0-100 Wetting agent 0-10 Antioxidant 0-3 vulcanization accelerator 0-5 Sulfur 0-2 Zinc oxide 0-5 Reinforcing agents include carbon black, wood flour, lignin, and various reinforcing resins such as'styrenedivinylbenzene, methyl acrylate-divinylbenzene, acrylic acid-styrene-divinylbenzene, and methyl acrylate-acrylic acid-divinylbenzene resins.

In general, any rubber plasticizers can be employed in these binder compositions. Materials such as Pentaryl A ('a mylbiphenyl), Parafiux (saturated polymerized hydrocarbon), (Zircosol-ZXH (petroleum hydrocarbon softener having a specific gravity of 0.940 and a Saybolt Universal viscosity at 100 F. of about 2000 seconds), di(1,4,1-trioxaundecyl)methane designated as TP-B and available from the Thiokol Corp., and dioctyl phtha-late are suitable plasticizers. Materials which provide rubber having good low temperature properties are preferred. It is also frequently preferred that the plasticizers be oxygen-containing materials.

Wetting agents aid in deflocculating or dispersing the oxidizer, Aerosol OT (dioctyl ester of sodium sulfosuccinic acid), lecithin, and Duomeen C diacetate (the diacetate of trimethylene diamine substituted by a coconut oil product) are among the materials which are applicable.

Antioxidants include Flexamine (physical mixture containing 25 percent of a complex diarylarnine-ketone reaction product and 35 percent ofN,N'-diphenyl-p-phcnylenediamine), phenyl-beta-naphthylamine, 2,2-methylenehis(4-methyl-6-tertbutylpheno-l), and the like. Rubber antioxidants, in general, may be employed or if desired may be omitted.

Examples of vulcanization accelerators include the carbamate type, such as N,N-dimethyl-S-tert-butylsulfenyl dithiocarbamate and Butyl Eight. Butyl Eight is a rubber accelerator of the dithiocarbarnate type supplied by the R. F. Vanderbuilt Company and described in Handbook of Material Trade Names by Zimmerman and Lavine, 1953 edition, as a brown liquid; specific gravity 1.01; partially soluble in water and gasoline; and soluble in acetone, alcohol, benzol, carbon disulfide and chloroform. Other types of accelerators include NOBS Special described as N-oxydiethylene benzothiazole-Z-snlfenamide, and available from American Cyanamid Company; and Santocnre NS described as N tert-butyl-2-benzothiazole sulfenamide, and available from Monsanto Chemical Company These two accelerators are described in Rubber Red Book, th edition, 1955-56, published by Rubber Age, New York, New York It is to be understood that each of the various types of compounding ingredients may be used singly or mixtures of various ingredients performing a certain function may be employed It is sometimes preferred, for example to use mixtures of plasticizers rather than a single material.

The various ingredients in the propellent composition may be mixed on a roll mill or an internal mixer such as a Banbury or a Baker-Perkins dispersion blade mixer may be employed. The binder forms a continuous phase in the propellant with the oxidant as the discontinuous phase.

Rocket grains are formed by compression molding, injection molding, injecting molding, or extrusion.

The curing temperature will be limited by the oxidizer employed in some cases but will generally be in the range between 70 and 250 F., preferably between 70 and 200 F. The curing time must be long enough to give required creep resistance and other mechanical properties in the propellant. The time will generally range from around three hours when the higher curing temperatures are employed to seven day-s when curing is effected at lower temperatures.

While this invention has been described using as the binder for propellent compositions a copolymer of a conjugated diene with a polymerizable heterocyclic nitrogen base of the pyridine series, such as vinylpyridine and various alkyl-substitutcd derivatives, it is to be understood that the corresponding quinoline and isoquinoline compounds are also applicable, i.e., vinylquinolines, vinylisoquinolines and various alkyl-substituted derivatives The binder composition was prepared using'a 90/10 butadiene/2-methyl-5-vinylpyridine copolymer having a Mooney value (ML-4) of 20. The binder composition was prepared in accordance with the following formulation:

Parts [by weight Butadiene/2-methyl-5-vinylpyridine copolymer 100 Carbon black, med. abrasion furnace 20 Benzophenone i 20' N,N-dirnethyl-S-tert butylsulfenyl dithiocarbarnateu 1 Sulfur 0.75 Zinc oxide 3 Aerosol OT 1 1 Flexamine 2 i 3 1 Dioctyl ester of sodium sulfosuccinic acid.

2 Physical mixture containing 25 percent of a complex diarylamine-ketone reaction product and 35 percent of N,N-diphenyl-p-phenylenediamine.

The various ingredients making up the propellants were mixed in a Baker-Perkins dispersion blade mixer. until a substantially homogeneous mixture was obtained with the binder forming the continuous phase in the final composition. Material from each composition was extruded into a sheet 0.25 inch in thickness from which tensile speciv Fresh propellant:

I Ballistic dataz Strand data:

r at 1,000 p.s.i.. i11./see. 1 0.177 0.163 n, pressure exponent 0.56 0. 54 Motor data:

r at 1,000 p.s.i., inJsec. 1 0.177 3 0.159 n, pressure exponent 0.56 3 0.52 0*, ft./sec. (characteris locity) 4, 070 3 4, 060 Smoke 4 4 3 9-12 Mechanical properties:

Ultimate stress (tensile), p.s.i 234 335 Ultimate elongation, percent 14. 5 15. 8 Modulus of elasticity, p.s.i V 4, 700 4,400 Propellant aged 2 weeks at 170 F.: Ballistic data-Strand data:

r at 1,000 p.s.i., inJsec. 0.187 n, pressure exponent 0. 56 Mechanical properties:

. .Ultimate stress (tensile), p.s. 214 228 Ultimate elongation, percent 16. 0 4. 8 Modulus of elasticity. p.s.i 2, 600 6, 900

Propellant aged 1 month at 170 I Ballistic data-Strand data:

r at 1,000 p.s.i., in./sec. 1 0.187 0.156 n, pressure exponent Z 0.57 0.52 Mechanical properties:

Ultimate stress (tensile), p.s.i- 349 124 Ultimate elongation, percent" 10.6 2. 9 Modulus of elasticity, p.s.i 5, 600 6, 000

1 Burning rate.

2 Pressure exponent in chamber pressure.

3 Average values from a series of 22 firings on grains from a series of batches all prepared in the same manner.

4 Percent attenuation of light beam in path of exhaust.

These data show that when using magnesium oxide tensile strength and elongation values remain high upon aging and that the modulus of elasticity is affected only slightly. The burning rate of the propellant is only slightly altered and a substantial decrease in smoke evolution is obtained by the addition of magnesium oxide.

Example II Propellants were prepared having the following compositions in parts by weight.

oxidizer (NHrNoa) 82.5 82. 5 82.5 Binder 15 17. 5 17. 5 15 17. 5 Miori blue- 2 2 2 2 2 MgO 0. 8 1. 0 0. 5 ZIlO 0. 3 0. 8 0. 3 0. 3

The binder employed in runs 1 to 5 above was prepared according to the following formualtion.

Butadiene/Z-methyl-S-vinylpyridine copolymer Carbon black, medium abrasion furnace 20 Benzophenone 20 N,N,-dimethyl-S-tert-butylsulfenyl dithiocarbamate- 1 Sulfur 0.75 Aerosol OT 1.0 Flexamine 3.0

The ingredients were mixedand specimens prepared as in Example I. All specimens were cured 24 hours at I r=aP., where a is a constant and P, is average Parts by weight 2. 170 F. Physical properties and ballistic data ofthe fresh and aged compositions are tabulated below.

from the group. consisting of ammonium dichromate, potassium dichromate, irn oxide, a complex cyanide of Run No 1 2. 3; 4 5' Code No 439.B53 628-A1 627A,1 493A-1 492-A-1.

Fresh propellant-z Strand burning data:

r at 1,000 p.s.i; (in;-./sec.) 0. 161 0 152 02155 0.154 0.153; n, pressure exponent 0.51 0. 50, 0.;49 01 53 0,55 Mechanical properties:

Ultimate stress (p.sdz) 211 274 219; 184 187 Ultimate elongation (percent), 25. 6 17.2 24. 1 234 9 25. 1 Modulus elasticity (p.s.i.). 2, 300 3, 038 1, 89 1, 800 2, 200 Propellant aged 1 week at 170F;:

Strand burning data:

r at 1,000 p.s.i. (injsee) 0.157 0; 160 0:151 n, pressure exponent 0; 51 0.58 0. 54 Mechanical properties:

Ultimate stress (p.s.i.) 274 245 Ultimate elongation (percent) 18.0 22. 8 Modulus elasticity (p.s.i.) 2, 100 1, A00 Propellnt aged 2 weeks at 170 F.:

Strand burning data:

r at 1,000 p.s.i. (in/sec.) 0. 157 0. 151 0.151 0. 161 0.153, n, pressure exponent 0.50 0. 52 0. 46 0.51 0. 54 Mechanical properties:

Ultimate stress (p.s.i.) 194 279 178 296 295 Ultimate elon ation (percent) 14. 7 10. 8 13. 4 16.0 19. 7 Modulus elasticity (p.s.i.) 2, 000 3, 108 1, 778 2, 400 2.000 Propell nt aged 4 weeks at 170 F;

Strand burning data:

r at 1,000 p.s.i. (in/sec.) 0.155 0.152 0. 162 0.152 n, pressure exponent 0.55 0. 51 0.53 0.55 Mechanical properties:

Ultimate stress (p.s.i.) 175 250 126' 309 291 Ultimate elongation (percent) 12. 1 9. 2 14. 0 13. 4 17. 3 Modulus elasticity (p.s.i.) 2, 100 3, 415 1, 398 2, 900 2, 100

The data reported in Example; II further demonstrate the improved aging characteristics resulting from the, addition of a minor amount of magnesium oxide to a propellent composition. These data, also indicate that both magnesium oxide and. zinc oxide are necessary to obtain the improved. againg characteristics. ample I contained zinc oxide in the recipe so that both zinc oxide and magnesium oxide were present in. the pro.- pellent formulation, identified as. run 1 of Example I.

Variations and modifications are possible within, the scope of the disclosure of the present invention, the essence. of which is: the discovery that the addition of a minor amount of magnesium. oxide to a:so1idrocket-fuell containing. a conjugated diene-heterocyclic nitrogenbase, copolymer binder results in surprising improvements in:

aging characteristics. of the rocket fuel and also resultsv in marked reduction by smoke evolution.

That which is claimed is:

1. A solid propellent composition oonsisting essentially of from 50 to 90 parts by weight of a, solid inorganic oxidizing salt and 10 to 50 parts by Weight of asul-furcured binder consisting essentially of a copolymer of a conjugated diene containing 4 to 6 carbon atoms per molecule and at least one substituted heterocyclic nitrogen base selected. from thegroup consisting of pyridine, quinoline, analkyl substitutedpyridine and an alkyl substituted-quinoline, wherein the total number of carbon, atoms. in. the. nuclear, alkyl substituents is not more. than 15. and wherein R is, se-- lected from the group consisting of hydrogen and a methyl radical, 0 to about 5-0 parts by weight per 100 parts of copol-ymerof carbon black, 0 to about 100 parts byweight per 100- partsof copolymer of a rubber plasticizer, O to about- 10 parts by- Weight P61.- 100;

and bindermixture of a burning rate catalyst selected" The binder. of Ex-.

3. The composition of, claim, 3, wherein the oxidizing salt is ammoniumperchlmfate.

4., The, composition of claim 1 wherein the, oxidizing salt,is potassium,nitrate.,

5;, The composition, of claim 1, wherein the oxidizing salt, is potassium perchlorate.

6. The composition of claim 1 whereinrthe copolymer is prepared from a mixture of monomers comprising 1,3-butadiene and 2-methyl-5 vinylpyridine.

7: The composition ofclaim 1 wherein the copolymer is prepared from a mixture of monomers comprising isoprene andZ-vinylpyridine.

8. The composition of claim 1 wherein the copolymer is prepared from a mixture of monomers comprising styrene and 4-vinylpyridine.

9. The composition of claim 1 wherein the burning rate catalyst is a complex cyanideof iron.

10. The. composition of claim 1' wherein the burning rate catalyst is milori. blue.

11. The composition of claim 1 wherein the burning rate catalyst: is ammonium dichromate,

12; The composition of; claim; 1 wherein the burning rate catalyst is, potassium dichromate.

131., The composition of. claim 1, wherein the burning rate catalyst is iron oxide.

14. In the manufacture of a solid propellant composition consisting essentiallyv offrom about 50 to about parts by weight of a solid inorganic oxidizing salt, about 10v to about 50 parts by weight of a zinc oxide containing, sulfur-cur ed binder comprising a copolymerv of a conjugated; diene containing4 to Q carbon atqms per molecule an at ha ens substituted heterocyclic nitrogen, base selected from the groupiconsisting of pyridine, quinoline, an alkyl substitutcd pyridine and an alkyl substitutedquinoline, wherein the total number of carbon atoms in the nuclear alkyl substituents is not more than 15 and wherein R is selected from the group consisting oi hydrogen and a methyl radical and 0 to about 15 parts by weight per 100 parts of oxidizing salt and binder mixture of a burning rate catalyst selected from the group consisting of ammonium dichromate, potassium dichromate, iron oxide, a complex cyanide of nickel, a complex cyanide of copper, and a complex cyanide 05E iron, wherein the ingredients are incorporated and cured, the improvement comprising admixing about 0.2 to about 5 parts by Weight per 100 parts of oxidizing salt and binder mixture of magnesium oxide No references cited.

Patent N00 3 010 814 November 28 19% Donald E0 Carr It is hereby certified that err ent requiring correction and that th corrected below.

or appears in the above numbered pat e said Letters Patent should read as Column 8 9 line 38 for the claim reference numeral 3" read l Sfigned and sealed this 3rd day of July 1962a (SEAL) Attest:

ERNEST w. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE, OF CORRECTION Patent N0o 3 OlO 81k November 28 196] Donald Eo Carr It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8 line 38 for the claim reference numeral 3" read 1 Sfggned and sealed this 3rd day of July 1962.

(SEAL) Attest:

ERNEST w. SWIDER. DAVID LADD Attesting Officer Commissioner of Patents 

1. A SOLID PROPELLENT COMPOSITION CONSISTING ESSENTIALLY OF FROM 50 TO 90 PARTS BY WEIGHT OF A SOLID INORGANIC OXIDIZING SALT AND 10 TO 50 PARTS BY WEIGHT OF A SULFURCURED BINDER CONSISTING ESSENTIALLY OF A COPOLYMER OF A CONJUGATED DIENE CONTAINING 4 TO 6 CARBON ATOMS PER MOLECULE AND AT LEAST ONE 